Anxiolytic cyclopyrrolone drugs allosterically modulate the binding of [35S]t-butylbicyclophosphorothionate to the benzodiazepine/gamma-aminobutyric acid-A receptor/chloride anionophore complex.

The influence of a number of anxiolytic cyclopyrrolone drugs, which include zopiclone and suriclone, on the binding of [35S]t-butylbicyclophosphorothionate (TBPS), to benzodiazepine/gamma-aminobutyric acid-A receptor/chloride anionophore complexes has been characterized in rat brain. Suriclone and its metabolites RP35,489 and RP46,166 are the most potent (IC50 approximately 3nM) inhibitors of [35S]TBPS binding thus far described, about an order of magnitude more potent than TBPS itself. The pattern of inhibition of [35S] TBPS binding by suriclone is distinctive; at approximately 10 nM there is approximately 50% inhibition of [35S]TBPS binding and inhibition "plateaus" at this level until suriclone concentrations exceed 1 microM. RP35,489 and RP46,166 display patterns of inhibition similar to suriclone. In saturation studies of [35S]TBPS binding, suriclone reduces the Bmax of [35S]TBPS-binding sites, with little or no effect on KD. Muscimol also displays a noncompetitive pattern of inhibition of [35S]TBPS binding, whereas inhibition by picrotoxinin appears competitive. [35S]TBPS dissociation is multiphasic and similar whether initiated by 10 microM TBPS or 10 microM picrotoxinin. By contrast, dissociation of [35S]TBPS is much faster (and nearly monophasic) when initiated by 10 microM TBPS/100 nM suriclone, 10 microM TBPS/1 microM muscimol, or 10 microM TBPS/1 mM pentobarbital. These results suggest that suriclone influences [35S]TBPS binding allosterically, at sites distinct from the TBPS/picrotoxinin recognition site. Inhibition of [35S]TBPS binding by suriclone varies regionally with a "plateau" at approximately 20% inhibition in the cerebellum, approximately 50% in the cerebral cortex, hippocampus and brain stem, and approximately 65% in the striatum and midbrain; by contrast, inhibition of [35S] TBPS by picrotoxinin, muscimol, and pentobarbital shows little regional variation. The inhibition of [35S]TBPS binding by suriclone is reversed by bicuculline [ED50 approximately 1 microM] in several brain regions examined. Bicuculline alone has little or no influence on [35S]TBPS binding in the cerebral cortex, hippocampus, and cerebellum, but produces a dose-dependent enhancement of [35S]TBPS binding in the striatum, midbrain, and hypothalamus. Regional differences in the effects of suriclone and bicuculline on [35S]TBPS recognition sites suggest possible heterogeneity in the coupling of cyclopyrrolone and bicuculline recognition sites to [35S]TBPS recognition sites in rat brain.